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If you've loaded up your smartphone or tablet with free apps and find your battery life taking a turn for the worse, it's not likely a coincidence.

The mobile advertisements on which free app developers depend to earn their profits are using up enough 3G or Wi-Fi connection to seriously drain your battery, say two researchers at Purdue University working with a Microsoft researcher.

In fact, Purdue's Abinhav Pathak and Y. Charlie Hu and Microsoft's Ming Zhang in their research paper said they believed that up to 75 percent of the energy that free apps like Angry Birds and The New York Times consume is spent marking your location and fetching suitable mobile ads to throw your way.

They used eProf -- "a fine-grained energy profiler for smartphone apps" -- to measure the performance of top apps from the Android Market, including Facebook, Angry Birds and Android browser, finding that in six of them "65% to 75% of energy is spent on third-party advertising modules."

The paper was first reported by the Web site New Scientist.

No Going to Sleep

The researchers noted that even as the app market explodes into a projected $38 billion industry by 2015, "their utility has been and will be severely limited by battery life," which makes optimizing power consumption crucial.

Still, using the eProf profiler, the team found that in the most popular apps, performing the task related to the purpose of the app, chess in a chess game for instance, consumes just 10 percent to 30 percent of power demand.

The apps prevent a phone from going into sleep mode with "wakelocks" that override the operating system's sleep policies, keeping the connection to the application program interface open for a short period even after use of the app is complete.

Apps that depend on other phone features such as the camera or GPS can keep those devices active after use, consuming power until they are turned off by another application, say the researchers.

Asynchronous Power Behavior

"Such asynchronous power behavior poses challenges to correctly attributing the energy consumption of the whole phone to individual program entities," write the researchers.

While the news may steer some phone users to more carefully scrutinize the apps they download, free apps will likely remain popular despite their drawbacks.

In 2011, 96 percent of smartphone apps were downloaded for free, according to market research firm IHS.

However, the firm found that "freemium" apps, those that entice users to a higher level of service once they get the initial free app, are very popular, representing nearly half of Apple iPhone's top-earning apps in 2011 and 31 percent of the top-earning Android Market apps.

"People are still open to receiving fee apps as long as they some sense of value -- especially those that are game related," said analyst Kirk Parsons of J.D. Power and Associates.